The present invention relates to methods for determination of the presence of Bactericidal/permeability-increasing protein in a body fluid sample including a blood sample. Bactericidal/permeability-increasing protein (BPI) is a cationic anti-microbial protein which has been purified from the azurophilic granules of human and animal neutrophils (Weiss et al., J. Biol. Chem., 253:2664 (1978), Elsbach et al., J. Biol. Chem., 254:11000 (1979). BPI binds to the lipopolysaccharide (LPS) component of the outer membranes of gram-negative bacteria (Gazzano-Santoro et al., Infect. Immun., 60:4754 (1992)). Recently, a recombinant form of human BPI (rBPI.sub.23) has been characterized and compared to that of native BPI. The rBPI.sub.23 fragment consists of the amino-terminal 23 kDa portion of holo-BPI and retains the LPS binding properties, as well as the anti-microbial activity, of native BPI (Gazzano-Santoro et al., J. Clin. Invest., 90:1122 (1992), Weiss, et al., J. Clin. Invest., 90:1122 (1992)).
BPI levels have not previously been accurately assayed in any body fluids. Because of the potential therapeutic use of rBPI.sub.23 and other BPI proteins and protein products, a sensitive and reproducible assay is needed to measure the presence and amount of BPI in body fluids. In particular, measurements of BPI in body fluids may be useful for diagnostic purposes. Pereira et al, J. Immunol. Methods, 117:115 (1989) discloses a competitive ELISA assay for the determination of BPI in crude granule extracts of human neutrophils. Pereira et al. also disclose that non-specific interactions of cationic proteins in an ELISA assay can be minimized by treatment with polyanions such as heparin or dextran sulfate. See also Pesce et al., J. Immunol. Methods, 87:21 (1986). However, the competitive assay of Pereira et al. is characterized by limited sensitivity. Accordingly, them remains a desire in the art for a more sensitive BPI assay capable of measuring endogenous BPI levels in mammalian body fluids. Also of interest to the present application is the disclosure of von der Mohlen et al., Abstract, 13th International Symposium on Intensive Care and Emergency Medicine, Brussels (March 1993) disclosing the results of assays for serum levels of BPI in patients with gram-negative sepsis and healthy subjects. The abstract disclosed that no BPI was detectable under the conditions of the assay in the serum of healthy subjects while circulating BPI was detected in all septic patients.